1. Field of the Invention
The present invention relates to control loader systems and methods which apply a load in response to a detected force. Specifically, this invention relates to a control loading system and method for generating a desired dynamic load such as may be employed in an aircraft flight simulation system.
2. Description of the Related Art
The need for controlling a motor or actuator in response to external parameters exists in production systems, robotics, and simulation systems, among others. The control of such actuators is critical not only in the static environment, but also in the dynamic environment, where an actuator's response varies as a function of the acceleration, velocity and position of the load coupled to that actuator.
A classic need for dynamic control of actuators is found in simulators, such as aircraft flight simulators. In aircraft flight simulators, a pilot will move a control stick in response to various visual and auditory stimuli. Since the goal of such simulation systems is to generate the apparent realism of the simulation, the dynamic response (e.g., the "feel") of the control stick must correspond to the vehicle, whether it be a helicopter, a commercial jet airplane, or a military aircraft.
Conventional flight simulators apply a controlled load on a target, such as a control stick and associated linkage, by using a force sensor located between an actuator of the simulator and the target. A diagram of such a prior art control loader is shown in FIG. 1. A hydraulic actuator 10 secured to a substrate 12 exerts a force in response to a control signal supplied to the hydraulic actuator 10 from a controller 18. A pilot force sensor 14 is coupled between the hydraulic actuator 10 and the load target 16 to which the actuator force is to be applied. The load target 16 includes, for example, a pilot stick, and the mechanical linkage associated with coupling the pilot stick and the pilot force to the pilot force sensor 14. At the same time, a pilot is also applying a force to the target.
In operation, the hydraulic actuator 10 exerts a force in response to the control signal from the controller 18. The force from the hydraulic actuator 10 is applied to one side of the pilot force sensor 14 which transmits the force of the hydraulic actuator 10 to the load target 16. Because the pilot force sensor 14 is coupled to the load target 16 to which a pilot may also be applying a force, the load target 16 exerts a reaction force upon the pilot force sensor 14. The force generated across the pilot force sensor 14 is output to the controller 18, which provides an appropriate control signal output to the hydraulic actuator 10 in response to the detected force. As a result, by establishing a predetermined system response characteristic in the controller 18, the control loader of FIG. 1 responds to the forces exerted on the load target 16 in an attempt to simulate, for example, the load exerted by an aircraft upon its pilot control stick in response to the load exerted by the pilot on the stick.
A fundamental problem in the conventional arrangement shown in FIG. 1 is that the pilot force sensor 14 is unable to distinguish between the force exerted by hydraulic actuator 10 and the force exerted by the pilot on the load target 16. The overall system response will be affected by limitations of the hydraulic actuator 10, the mechanical couplings, and system factors such as inertia, damping, vibration, and forces which, like a spring, are dependent upon relative position. In a static system (e.g., the hydraulic actuator 10 and the load target 16 have no acceleration), the systemic limitations are not apparent so that the controller 18 is able to accurately control the system.
In a dynamic system, in which forces applied to the load target are changing, because the force exerted by the hydraulic actuator 10 cannot be distinguished from the force exerted by the pilot, the system of FIG. 1 cannot correct for systemic limitations such as inertia, damping, and vibration within the hydraulic actuator 10 or its associated mechanical linkages. As a result, the dynamic response is poor, and any attempts to use the prior art system in an aircraft simulator results in an aircraft control stick which gives a poor "feel" to the pilot who moves the control stick.
The control loader of FIG. 1 has the additional problems associated with using the hydraulic actuator 10. The hydraulic system tends to develop leaks, thus affecting the reliability and accuracy of the system. Attempts have been made to replace the hydraulic system with a motor, but because the pilot force sensor 14 is unable to distinguish between the force exerted by the motor and external forces acting on the load target, problems arise in eliminating ripple caused by the motor.
A further disadvantage to the prior art system is that because the controller 18 cannot compensate for the inertia or damping in the hydraulic actuator 10 or the associated mechanical linkages, the prior art system cannot be used for simulator systems wherein the stick of the aircraft being simulated has less inertia than the simulator's mechanism. This is because the prior art control loader system cannot take into account its own inertia. For example, while the system of FIG. 1 may be adequate to simulate a military fighter craft, which has a relatively heavy "feel", the inertia of the system may be too great for simulating a helicopter, which has a relatively light "feel". Consequently, different control loading systems are required having force and inertial characteristics which match the desired application.